The proprioceptive system converts information from receptors in muscles, skin and joints for the purposes of perceiving both the internal state of the neuromuscular system (e.g. the position of limbs, the forces generated by muscles) and the properties of objects (e.g., weight, stiffness) encountered in the external world. There is considerable kinematic ambiguity in these afferent signals as mechanoreceptors in muscles, skin and joints do not simply encode a single stimulus but respond to a number of variables both mechanical and temporal. Despite this ambiguity, the proprioceptive system can still extract the necessary information, such as joint velocity or angular position, from the sensory input and use this both to control and perceive muscle force and limb movements. The long-term goal of the present research is to understand The principles and mechanisms underlying these perceptual processes and to determine the commonalities in information processing shared by the proprioceptive system with other sensory modalities whose inputs arise externally. The present proposal uses human psychophysical techniques to address these issues in several series of experiments that will examine the nature and extent of spatial summation of forces in the hand using the contralateral limb-matching procedure, the temporal processing of limb movements and the motor and sensory mechanisms involved in perceiving derived percepts such as stiffness. The movement studies will initially focus on determining whether frequency selectivity, a property of many sensory modalities, characterizes proprioceptive processing. This will be measured in terms of tuning curves and peripheral filtering processes (i.e. critical bands). Related studies will determine what factors influence the perception of movement velocity under active and passive conditions, during fast and slow movements and when cutaneous signals are masked. The final series experiments will determine how subjects perceive properties such as stiffness, viscosity and inertia on the basis of muscle force and limb displacement signals, and whether, as predicted, the motor strategies used to derive this information differs for each of these variables. This research program will elucidate the basic operations of the human proprioceptive system, improve our understanding of normal perceptual functioning and provide a basis for interpreting neuromuscular and neurological disorders that impact human movement control.